Pharmaceuticals with very poor water solubility present formulation problems due to their slow rate of dissolution. Their efficacy can by severely limited and large interindividual variations of absorption can occur. Examples of drugs with very low solubility are some substituted dihydropyridine compounds such as nifedipine and felodipine. The mentioned dihydropyridines are commonly classified as calcium antagonists, which are widely used for the treatment of cardiovascular disorders such as ischaemic heart disease and arterial hypertension. One of the mentioned dihydropyridines, namely felodipine, has a solubility of only 0.5 mg/l in water. Other examples of drugs with very low solubility are griseofulvin, digoxin, oxazepam, phenytoin and cyclosporine.
Several ways to increase drug absorption have been described in the prior literature. One way is described in DE-A No. 3024858, where a sparingly soluble substituted dihydropyridine, nicardipine, is used in its amorphous form in order to obtain increased absorption of the active compound from the intestine. Another way is described in EP-A No. 47899, where very small crystals of a practically insoluble dihydropyridine, nifedipine, have been used in order to increase the extent of the bioavailability. These methods and others are also described in "Techniques of solubilization of drugs", Ed S. H. Yalkowsky in Drugs and the pharmaceutical sciences, Vol. 12. Of particular relevance to the present invention is that surfactant solubilizing agents may be employed in order to increase the bioavailability of the drugs with very ow solubility. It is stated that the improvement of absorption properties can be ascribed to three processes: (1) increased wetting, (2) increased permeability of membranes and (3) solubilization. The cited publication describes several examples and serves as a good review of the state of the art concerning the solubilizing of drugs, especially in order to increase the bioavailability of drugs with very low solubility.
From DE-A No. 3400106 controlled release preparations are known containing one or more natural,partially synthetic or synthetic polymers, one or more lipophilic and/or hydrophilic solvents or thickeners together with one or more pharmaceutically active compounds. In the examples it is described to use a solubilizer in an amount by weight to the active compound which is much less than 1:1.
In the medical treatment of various diseases, e.g. in the cardiovascular, gastrointestinal and chemotherapeutic field, it is an advantage to have a constant concentration of the administered drug in the blood. Thus an extended release of the drug from the pharmaceutical preparation is wanted.
It is important that the extended release preparation delivers the amount of drug needed to maintain an adequate and even effect during the entire therapeutic dosage interval. This usually means that the drug should be delivered at a constant rate to give an even concentration of administered drug in the blood. This is of specific importance for drugs having a small therapeutic index, that is a small difference between effective and toxic concentration. A delayed and constant release of the drug will also be of importance for locally irritating drugs having potential risk of causing gastrointestinal disturbances when present in large local concentrations or for drugs having a short elimination half-life. In the latter case a less frequent administration and thus better patient compliance (cf. Hayes R. B. et al. Clin.Pharm.Ther. (1977), 22, p. 125-130) may be obtained with extended release preparations compared with conventional dosage forms.
A drug in extended release form is generally given via the oral route. The preparations should preferably give an extended and reproducible release of drug and contribute to a reproducible absorption, have no toxic or irritating constituents and be suitable also for high dosage drugs. Conventionally, extended release is achieved by controlling dissolution and/or diffusion of medicament from the dosage form. Several materials are employed for this purpose e.g. waxes, fatty materials, polymers, natural, synthetic and semisynthetic gums. Among the gums, hydroxypropyl methylcellulose (HPMC) constitutes an important class because of its pH-independent properties as well as its semisynthetic origin. A review of cellulose ethers in hydrophilic matrices for oral controlled release dosage forms is given by Alderman D. A. Int. J. Pharm. Tech. & Prod. Mfr (1984), 5(3) 1-9. The chemical treatment of HPMC to generate a desired constitution and the use of these qualities are disclosed in U.S. Pat. Nos. 3,087,790, 4,226,849, 4,357,469 and 4,369,172. SE-A-8008646-5 describes a combination of HPMC and hydroxypropyl cellulose which is used to control the release rate of a pharmaceutically active compound.
When a hydrophilic matrix is used the soluble polymer forms a gelatinous layer around the tablet after the exposure of the tablet to gastro-intestinal fluids or saliva. The release of the drug is limited by the rate of water penetration into, and diffusion of drug through, the gel formed (Bamba et al. Int. J. Pharm. (1979), 2, 307). Erosion of the gel structure is also an important release mechanism of a drug from the system. The polymers used have to hydrate rapidly in order to protect the tablet from fast dissolution (Alderman 1984).
The rate of absorption of a drug with very low solubility into the circulation from the intestinal tract is closely related to the rate of dissolution. Since a low dissolution rate generally results in a low extent of bioavailability it is difficult to decrease the rate of absorption, i.e. increase the duration, without at the same time lowering the extent of bioavailability.